What is an arachnoid cyst?

An arachnoid cyst is an entirely benign condition characterised by a collection of normal cerebrospinal fluid, which is the liquid in which the brain and spinal cord floats. The term arachnoid refers to the web-like appearance of one of the layers of the linings (meninges) of the brain and spinal cord. Arachnoid cysts can rarely occur as part of the early development of the brain when the arachnoid forms a pocket of cerebrospinal fluid (CSF) which does not communicate with the rest of the brain or spinal cord.

What problems can an arachnoid cyst cause?

Arachnoid cysts form early in brain development and do not impact on the normal growth and maturation of the brain. The vast majority will then remain unchanged throughout life without affecting the adjacent brain in any way. As a result, most arachnoid cysts are diagnosed on a CT or MR scan of the brain done for other reasons, and are of no significance.  Rarely, the arachnoid cyst might enlarge and press on some of the structures of the brain, thereby producing symptoms.  These symptoms will be related to the area of the brain it presses upon.  

We would wish to emphasise that in the vast majority of cases, an arachnoid cyst is an incidental diagnosis made after a scan and will never cause any problems. 

Should an arachnoid cyst be treated?

The vast majority of arachnoid cysts will not require treatment or follow-up. This is certainly the case if they are picked up as an incidental finding without any associated symptoms. It is, however, often helpful to discuss this with a neurosurgeon with expertise in the condition.  Having consulted such a neurosurgeon, most people can be reassured and no further follow-up is required.

How are arachnoid cysts treated?

As described above, surgery for arachnoid cysts is rarely required.  In the extremely unlikely event that a cyst is causing symptoms, the recommended treatment would be surgery.

The principle of this surgery is relatively straightforward and involves opening of the arachnoid cyst wall so that it communicates with the rest of the CSF space. This thereby decompresses the cyst and allows the symptoms caused by the mass (volume) effect of the cyst to be relieved.  The precise nature of the surgery depends on the location of the arachnoid cyst, and can be done either by a using an endoscope through a small hole made in the skull, or by a craniotomy (opening of the head).